Through southern and central Wisconsin the eastern tiger swallowtail is a colorful beauty.

June 2008

A tale of two tigers

Swallowtail butterflies are easy to recognize, but the ranges of two similar species overlap in Wisconsin.

Anita Carpenter

High overhead, a big yellow butterfly with bold black stripes and two long black tails floats by. It drops in for a visit on a lavender phlox and with wings still fluttering, sips a nectar meal. If you approach for a closer look, the wary insect takes flight and all too quickly is up, away and gone. The beautiful, black striped butterfly is easily identified as a tiger swallowtail. The challenge is, two tiger swallowtail species grace the open woodlands and nearby fields in Wisconsin. Before 1991, both species were classified as the eastern tiger swallowtail Papilio glaucus. However in that year, lepidopterists decided that the slightly smaller northern form that had been considered a subspecies should be elevated to a separate species based on some biological differences. Thus the Canadian swallowtail, Papilio canadensis, was officially recognized.

The two species look similar but have some subtle wing pattern differences. Insect size and location are a more defining distinction. The 3 1/2-4 inch eastern tiger swallowtail flies throughout the eastern United States from the gulf states northward to southern Wisconsin. The Canadian tigers, 2 5/8-3 1/4 inches in length are the big creamy yellow butterfly of the Northwoods whose range extends into Canada where it is the most common tiger swallowtail present. The ranges of the two species overlap in central Wisconsin roughly along Highway 21. Look carefully if you are identifying butterfly species in this area as they don't recognize boundaries!

For visual clues, look for these tricky differences in their wing patterns. Particularly, look at the yellow band just inside the outer edge on the underside of the front wings (forewings). That's difficult to do on a butterfly that seems to constantly move. On the eastern tiger swallowtails, a series of disconnected yellow spots on the black band decorate the wing, but there are variations. The Canadian tiger swallowtail has a continuous yellow band. Also on the underwings, the black band along the inner wing margin is wider on the Canadian swallowtails. Additionally, it seems to me that the Canadian tigers have more black hairs covering their bodies and look hairier than their southern cousins.

The Canadian tiger swallowtail is a little smaller than the eastern tiger.

Life histories of the two species are similar. Females lay spherical eggs singly on host plants. Eastern tiger caterpillars in Wisconsin eat cherry and ash, while Canadian tiger swallowtails prefer birch, aspen, willows and cherry. Between their early molts, caterpillars of both species look like bird droppings to protect themselves from predation. The mature caterpillars are a beautiful leafy green color with two black-centered yellow eyespots on a swollen thoracic segment. The larvae feed at night and rest during the day rolled up in leaf shelters to make them hard to find. The chrysalides hang in trees and overwinter.

Both species emerge from their chrysalides from mid-May through early June and are flying about the same time. The adult Canadian tigers only have one flight period from May into June and early July. Eastern tigers have two flight periods per summer. Individuals from the second brood are flying in August. So if you see a tiger swallowtail flying in later summer, you can be sure it is an eastern tiger.

When you are taking a close look at the eastern tigers, be alert for another interesting aspect of their biology. A small number of the eastern tiger females appear more black than yellow. Occasionally you will see a totally black swallowtail drifting across your path. The true black swallowtails (Papilio polyxenes) show a bright yellow band on the males and a hint of a yellow band on the females bisecting the wings. If you see a black swallowtail without such bands, it is likely the black female form of the eastern tiger swallowtail. It's pretty rare to see these, as the number of melanistic or black eastern tigers decreases significantly as you journey north from the gulf states up toward Wisconsin. In the southern states, these female black eastern tigers mimic the pipevine swallowtail that other animals find distasteful. The pipevines don't live this far north, and the number of black eastern tiger swallowtails found here is also very low.

Even though the swallowtails are big, elegant butterflies that are easy to spot, take the time to take a closer look to see if you can identify which species you are seeing. And if you can't get that close, just enjoy them for the grace and beauty they lend to our lives and landscape.

Anita Carpenter keeps a close eye on creatures large and small near her Oshkosh home.